The maximum differential bubble pressure method, employing two orifices of different diameters immersed in the body of a fluid was tested and proven to be adaptable for measuring in a pressurized environment over fifteen years ago as described in U.S. Pat. 4,416,148, up to a nominal 10 psig pressure, at 25 degrees Celsius. Electronic hardware peak detection for fluid surface tension measurement with the modified maximum differential bubble pressure technique has proven satisfactory for non-viscous fluids, and fluids tested under non-pressurized conditions. Hardware peak detection is limited to transducer output signals that are unipolar (positive) in value, nominally between 0 to 5, or 0 to 10 Volts DC. Hardware peak detector circuits will, however, false trigger on a zero crossing.
Electronic hardware peak detection circuits have a number of further limitations when certain pneumatic conditions change the differential pressure waveform by generating false peaks that trigger the hardware peak detector. Hardware peak detectors can false trigger (see FIG. 4) on pressure signal fluctuations that are caused by capillary action when 0.1 mm I.D. and larger orifices are used in the small orifice position. As the viscosity of a liquid increases, there is increased hydrodynamic resistance of the liquid against a moving bubble. Very viscous fluids, and fluids with high suspended solids concentration, cause electronic peak detectors to false trigger. Greater pneumatic pressures required to overcome the increased hydrodynamic resistance at an orifice can cause unstable or noisy waveforms.
Lowering the amplitude of the differential pressure waveform will reduce the amplitude of the false peaks proportionately so that the electronic hardware peak detector no longer trigger on the false peaks; however, this can lower the amplitude of the waveform in lower surface tension fluids to the point where they no longer trigger the electronic peak detector. In this situation, it is no longer possible to calibrate the instrument in a low standard calibration fluid, as for example, alcohol.
The electronic hardware peak detector will also false trigger on waveform noise oscillations that result when the measured test fluid is pressurized. Mass flow controllers, required to operate in an increasing pressurized environment, cause a maximum bubble pressure waveform that becomes increasingly unstable between bubbles (FIG. 6). Large oscillations occur following the release of each bubble before the system stabilizes and the next bubble is blown.
In a non-pressurized environment the bubble rate remains constant once the flow rate is set with mass flow controllers. However, in an increasing pressurized environment, although the maximum bubble pressure remains constant and therefore surface tension remains constant, bubble rate will decrease (slow down) with increasing pressure (see FIGS. 4 and 6).
Electronic hardware peak detection circuits are further limited in responding to various amplitude and frequency changes of the maximum bubble pressure waveform and waveform shapes will change as bubble rate is changed, and as fluid viscosity increases. At one bubble per second, the waveform flows a sawtooth configuration (FIG. 4) where a linear positive slope follows the increase in pressure as the bubble is formed up to its maximum bubble pressure point. When a bubble releases there is a sharp drop (negative downslope) followed by a momentary back pressure and capillary action before the pressure equalizes inside the tube and the next bubble begins to form. The positive slope is commonly referred to as the "surface age" of the bubble while the rest is commonly referred to as "dead time" (FIG. 5).
An ideal hardware peak detector should track only the surface age (positive) portion of the sawtooth wave until it reaches a valid maximum, capture that maximum value, trigger a reset signal by detecting the subsequent drop (negative downslope), and then track the next valid peak.
The dead time of a sawtooth waveform is finite and depends on the rheology of the fluid, the diameter and configuration of the orifice, and the pressure characteristics of the mass flow controllers. As bubble rate increases, dead time becomes a greater proportion of the peak-to-peak bubble interval time. At one bubble per second (FIG. 4) the surface age typically is in excess of ninety percent of the bubble interval, while at thirty five or more bubbles per second, the surface age can be less than ten percent of the bubble interval (FIG. 5).
Mass flow controllers are set for a specific flow rate when an instrument is set up and calibrated; however, bubble rate will change if surface tension of the fluid changes, even though flow rate stays fixed. A peak detector must be flexible enough to cover all possible bubble ranges. For example, a flow setting that produces one bubble per second in water, with surface tension in the 70+dynes/cm. range, produces more than three bubbles per second in alcohol, with surface tension typically in the 20 plus dynes/cm range. The waveform amplitude in alcohol is much smaller due to lower surface tension of alcohol. Electronic peak detection circuits lack capability to ignore various noise oscillations and signal combinations as described.